1. Field of the Invention
This invention pertains to the conversion of picture information on a motion picture film into a video signal and, more particularly, to a film scanning apparatus commonly known as a telecine scanner, which is used for playback of a motion picture film for television production and programming.
2. Background Art
A telecine scanner is ordinarily balanced "open gate" prior to scanning a motion picture film. This requires that the telecine scanner be adjusted so that when there is no film or filters in the light path from the light source through the film gate of the scanner to the image pickup devices, the red, green and blue video signal outputs are of equal amplitude. The equal signal amplitudes define a reference level corresponding to white light input, or some equivalent to white light input. Ordinarily the gain controls of the video amplifiers coupled to the image pickup devices are adjusted to produce such equal amplitudes, which are sometimes referred to as "100% video" output levels.
"Open gate" balancing is especially adapted for motion picture print film, since equal amounts of the positive-reading density information in the film will thereafter affect light transmission equally in each color. Motion picture print film, moreover, has been traditionally preferred for telecine scanning because positive prints, besides being readily available, are already color balanced for direct viewing and require fewer color corrections than a negative film. However, the making of a positive film print from the original negative film requires at least one extra processing step, which results in some degradation of the image as well as color saturation of the resulting print relative to the negative.
It would, consequently, be desirable to use a negative film . . . as well as a positive film . . . directly in a telecine scanner. A direct substitution of a negative film for a positive film is ordinarily unpromising since a negative film has an inherent base density that provides unequal transmission for like components of red, green, and blue light input to the negative film. The base density includes, in the case of negative film, the density of unexposed areas of the film, including the densities of the support and suspending gelatin, the fog density produced on development without exposure, and the unequal densities in red, green, and blue due to the inherent color mask used in a negative film to cancel unwanted dye absorption. This overall unequal density distribution becomes the point from which net densities produced by exposure and development are measured. The base density (also called the minimum density or "D-min.") of a negative film, for example, is about 0.3 units in red, 0.6 units in green and 0.9 units in blue. Such a typical negative material, therefore, blocks light transmission by about 50 % in red, 75% in green and 88% in blue--without any image information yet in the film. The penalty for balancing a telecine scanner "open gate" is thus taken out in signal to noise for negative film, that is, signal to noise performance is reduced by a factor of 2 in red, by 4 in green, and by 8 in blue when scanning a negative film. This is clearly unacceptable.
Prior art attempts to correct for this imbalance generally depend upon either equalization of the light transmission of the negative film in red, green, and blue by adding filters into the light path (see, for example, U.S. Pat. No. 4,009,489), or by electronic matrixing of the red, green, and blue signals (see "The Interface of Color Negative Film and Telecine," by Karel Staes and Walter Markie, SMPTE Journal, March 1983, pp. 303-307), or by some combination of both (see "FDL60-An Advanced Film Scanning System," by Dieter Poetsch, SMPTE Journal, March 1984, pp. 216-227). Color equalization filters are inserted into the light path during film scanning so as to reduce red and green light transmission to match the blue light transmission of the negative film. When this method is utilized for a typical negative film, the video output signal amplitude is greatly reduced from its original "100% video" level. In order to restore the level needed for video processing, the gain of the image pickup amplifiers is uniformly increased, thus raising the typical noise level in the output signal. Alternatively, if optical equalization filters are not used, the individual gains of the image pickup amplifiers must be increased by different amounts, also increasing noise.
In U.S. Pat. No. 4,009,489, the output video signals are balanced for negative scanning by inserting optical equalization filters into the light path that optically match the red image light output to the green image light output. Then the electrical gain of only the blue light channel is increased in order to balance the output video signals. A "100% video" level is then restored by increasing the illumination. A related technique was disclosed in the Poetsch article (SMPTE Journal. March 1984). On changeover to negative film scanning (from positive print scanning), a cyan correction filter is inserted into the light path to correct for the orange mask in negative film. The black-and-white levels of the RGB signals are then adjusted for different negative materials.
Apart from the problem of handling the base density of negative film, a telecine scanner has to process films which are not primarily designed for the spectral sensitivities of its pickup devices, sometimes using imperfect light sources and less than ideal receptors. Electronic masking is used to correct for these factors. In the Poetsch article, a switchable RGB matrix provides the choice of two positions for positive film material and negative film material. Electronic masking oftentimes also attempts to correct for the mask dyes it "sees" in negative film. In the Staes and Markie article (SMPTE Journal, March, 1983), special matrix coefficients are developed to further eliminate the film mask.
Conventional telecine scanners thus treat negative scanning as an adjunct of positive scanning. Correction is attempted by either increasing the gain of the preamplifier section (with or without correction filters added) or by increasing the amount of electronic matrixing, or by both. Merely increasing gain inevitably reduces the signal in relation to noise. More matrixing means more signal contribution in each color channel from the other color channels, i.e., more cross-channel "noise". It is clear that color negative motion picture film is not currently processed in the most effective way in a telecine scanner.